Field of the Invention
The present disclosure relates generally to the field of optical and medical devices, and more specifically to an apparatus and method for optical communication, biological sensing, and chemical sensing.
Background of the Invention
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Silicon photonics has been attracting intense interest in the last decade due to its great potential in realizing low cost photonic chips with the readily-available CMOS manufacture technology. However, the fact that silicon does not have either a direct band gap or a second-order nonlinearity makes it a great challenge to generate or control photons. Additionally, the large refractive index contrast weakens the photon-matter interaction and makes silicon a less attractive sensing platform. The lack of direct band gap or second-order nonlinearity may potentially be solved by hybrid integration of silicon and functional cladding material and this issue along with the issue of the large refractive index may be solved by a waveguiding structure that facilitates the photon-matter interactions.
Plenty of structures, such as slot waveguides and photonic crystal waveguides have been exploited to enhance the photon-matter interaction. Subwavelength photonic crystal waveguides, comprised of periodically arranged high index and low index materials with a pitch of less than one wavelength, have received considerable attention in recent years. Bloch modes may be supported by this periodic arrangement of silicon pillars and cladding material, and therefore photons may propagate in theory without being attenuated by the discontinuity of the mediums. Subwavelength photonic crystal waveguides provide another dimension of freedom to precisely control a few important waveguide properties such as refractive index, dispersion, and mode overlap volume, which are determined by the materials comprising the waveguides as demonstrated previously. The control of these properties enables significant improvements over conventional waveguide based devices such as grating couplers, directional couplers, sensors, filters, and modulators.
One critical problem remaining unresolved is the large loss of the subwavelength photonic crystal waveguide bends. For instance, a 10 μm radius 90° bend has an insertion loss of ˜1.5 dB. To avoid the substantial loss introduced by a subwavelength photonic crystal waveguide bend, the subwavelength photonic crystal waveguide is tapered to conventional strip waveguides before reaching a bend and further tapered back to subwavelength photonic crystal waveguides afterwards. Although the strip waveguide bends can significantly reduce the loss, the taper adds additional loss and wastes the precious silicon chip surface. Therefore, to achieve the goal of building integrated photonic systems with entirely subwavelength photonic crystal waveguides, a low loss and small bend radius subwavelength photonic crystal waveguide bend is highly desirable. In addition, the low loss bend is an essential component for high quality factor subwavelength photonic crystal waveguide ring resonators, which can be used for optical modulators, switches, filters, and sensors.